1. Field of the Invention
The present invention relates to an automatic focus adjustment camera which is capable of sequential photography, and to a method of performing automatic focus adjustment for a camera, suitable for practice during sequential photography.
2. Description of the Related Art
An automatic focus adjustment camera is per se known, in which focusing of the optical system for photography is accomplished by an image of the object to be photographed being cast by a focus detection optical system upon an image sensor of a per se known charge accumulation type, by appropriate calculation processing being performed upon the output of this image sensor--thereby detecting the amount of defocusing of the image at a previously determined focal plane of the photographic optical system--and by a focusing lens being driven according to this detected defocusing amount, so as to bring the image of the object to be photographed into focus at the focal plane.
Further, a sequential photography device for a camera is per se known, which operates the camera mirror and shutter while progressively winding on the film through a sequence of film frames, so as to perform sequential photography.
However, if such a sequential photography device is fitted to an automatic focus adjustment camera of the type described above, then, when sequential photography is performed in the automatic focus adjustment mode, since during the film frame interval between the completion of the shooting of each film frame and the shooting of the next film frame the focus adjustment process of calculation processing etc. is performed in the same manner as during normal photography in which a single film frame only is shot, the problem arises that high speed sequential photography becomes impossible, due to the length of this focus adjustment time period (equal to the time period required for focus adjustment calculation processing plus the time period required for actually driving the photographic lens) which overflows the desired time interval between shooting consecutive film frames.
This problem will now be explained in detail with reference to FIGS. 14A, 14B, 15A, and 15B, which refer to the prior art.
FIG. 14A is a time chart showing the sequence of operation of a prior art camera when performing sequential photography. For the shooting of one film frame, the following actions are performed in sequence: (1) the mirror is raised; (2) the shutter is operated and exposure is performed; (3) the mirror is lowered; and (4) the film is wound on and the shutter is recharged. And, in order to perform sequential photography, this operational sequence is repeatedly executed. Further, FIG. 14B is a time chart showing the automatic focus adjustment sequence (hereinafter abbreviated as the AF sequence) during sequential photography in synchronization with the operational sequence shown in the FIG. 14A time chart. When the lowering of the mirror (in the FIG. 14A time chart) has been completed, it is possible for the charge accumulation operation of the image sensor for focus detection (in the FIG. 14B time chart) to be commenced. Next, when this charge accumulation operation has been completed, it is possible for the signal representing the image of the object to be photographed to be read out of the image sensor, so that the calculation process for focus detection which determines the amount of defocusing of the optical system for photography can be commenced. And next, when this calculation process has been completed and an lens drive amount for driving the focusing lens has been derived according to the calculated defocusing amount, it is possible for driving of the focusing lens to be commenced. Furthermore, if in this AF sequence (in the FIG. 14B time chart) the operation of driving the focusing lens has not been completed by the time point in the operation sequence (in the FIG. 14A time chart) for the starting of exposure and shutter operation for the next film frame, then the driving of the focusing lens is discontinued at this time point.
When sequential photography of a moving subject is being performed, it is necessary to drive the focusing lens for each film frame, in order to achieve satisfactory focusing operation. FIG. 15A is a time chart showing the operational sequence of a prior art camera when performing high speed sequential photography, and in this figure the time period allotted for performing the film winding on and shutter recharging operation is shorter than in the time chart of FIG. 14A. If an attempt is made to implement an AF sequence of the type shown in FIG. 14B together with this type of camera operational sequence during high speed sequential photography, it becomes impossible to maintain the time period for driving the focusing lens at the length shown in FIG. 14B; in other words, not enough time is available for proper focusing operation. The only way in which, with an operational sequence for high speed sequential photography of the type shown in FIG. 15A, the time period for driving the focusing lens can be maintained at the length shown in FIG. 14B, is for the time period for performing the calculation process for focus detection to be shortened, as shown in FIG. 15B.